Crystengcomm

CrystEngComm View Article Online HIGHLIGHT View Journal | View Issue Photo-induced motion of azo dyes in organized media: from single and liquid crystals, to MOFs Cite this: CrystEngComm,2016,18, 7204 and machines O. S. Bushuyev, T. Friščić* and C. J. Barrett* The field of photo-mechanical effects is a burgeoning branch of materials science dealing with the direct transformation of light energy into mechanical motion. While the research in the field has historically fo- cused on polymeric materials, the past decades have seen the rapid emergence of crystalline, photo- mechanically active materials capable not only of converting light into mechanical motion, but also cou- pling such behaviour with other materials properties, e.g. microporosity in metal–organic frameworks (MOFs). This Highlight article focuses on the rapidly emerging, new area of photo-mechanical materials based on crystalline azobenzenes. The discovery of photo-mechanical motion in a needle-shaped crystal of an azobenzene led to an explosion of research and new developments in less than 5 years, revealing Received 13th May 2016, new types of photo-mechanical behaviour, crystal engineering routes to easily create libraries of crystalline Accepted 14th June 2016 photo-mechanical materials, in situ and real-time studies of structural changes during photo-mechanical effect using X-ray diffraction, and the discovery of new supramolecular interactions enabling the engineer- DOI: 10.1039/c6ce01128d ing of photo-mechanical azobenzene crystals in terms of molecular stacking, as well as crystal www.rsc.org/crystengcomm morphology. Introduction Merian reported curious macroscopic motions of then-new nylon fibres dyed with azobenzene, when they were exposed There has been growing interest in using small molecules as to light. In those observations, the irradiated nylon filaments dyes and colourants for thousands of years now, from the shrank upon irradiation with a xenon daylight lamp,3 the earliest urges of humans to colour our bodies, cave walls and shrinking resulting from the reversible trans ↔ cis geometric Published on 14 June 2016. Downloaded by McGill University 02/12/2016 20:21:50. clothing, and then later our food, shelters, and artistic isomerization of the molecules bearing the azo chromophore canvases. More advanced applications of chromophores then (Fig. 1a). The energy absorbed by the molecules in the pro- developed over the past few centuries in early chemistry labo- cess of isomerization was transferred, although with low effi- ratories, in the form of visible sensors of local environments ciency, to the surrounding polymer matrix, driving an overall such as pH paper and titration indicators, where colours contraction of under 0.1% of the original length. Further re- switched with chemical change. Azobenzene in particular search into this curious phenomenon allowed the magnitude then started to emerge as a prime choice for such of contraction to be gradually increased to up to 1% of the – switching.1 7 A third wave of dye molecule development was original length, simply by increasing the loading of the seen in recent decades, where optical, electro-optic, and dye.4,5 photonic properties could be switched rapidly and reversibly Following from these original observations of photo- with light, to store and process information optically, and mechanical effects in textile fibres, much of the early research where azobenzene once again featured prominently.1 A most was concentrated on amorphous polymers, which later were recent area of azo dye research, however, is the control of found to be less effective in the transduction of energy of light macroscopic mechanical motion directly with visible light, into mechanical motion. Ordered materials, such as liquid akin to the reversible shape-change of retinal in rhodopsin crystalline elastomers (LCEs), proved superior in actuation when it absorbs a single photon and isomerizes, enabling vi- force and response speeds, and offered the further advantage sion.2 Such photo-mechanical effects were in fact observed of control over the direction of actuation via polarization.7 Ul- first 50 years ago during studies on textile processing, when traviolet (UV) irradiation of azobenzene-containing LCEs was found to disturb the mesogen alignment in the surface layers ↔ Department of Chemistry, McGill University, 801 Sherbrooke Street West, of the film, due to trans cis isomerization generating asym- Montreal, H3A 0B8, Canada. E-mail: [email protected], metric strain, causing bending of the film in a predetermined [email protected] direction. 7204 | CrystEngComm,2016,18, 7204–7211 This journal is © The Royal Society of Chemistry 2016 View Article Online CrystEngComm Highlight – Moving fully to single crystals in principle offers an even of metals,11 13 that the large steric demand of azobenzene faster and more efficient energy transduction owing to their isomerization would make such a process near-impossible in higher order and rigidity.8 This Highlight focuses on the new a densely packed crystalline environment. However, the ex- observations of motility of azobenzene chromophores in sin- plosion of research activity in photo-mechanically active crys- gle crystals and as parts of open metal–organic frameworks talline systems, inspired to the greatest extent by the seminal (MOFs), which offer attractive material pathways to construct investigations of the Irie group on light-induced switching of – complex functional architectures activated by visible light.9,10 diarylethenes in single crystals,14 17 led to a recent re- investigation of the possibility of azobenzene isomerization Azobenzene isomerization in crystals in crystals and, therewith, crystalline azobenzenes as photo- mechanical solids. A major breakthrough in the development Trans ↔ cis isomerization for reversible photo-mechanical of crystalline azobenzene photo-mechanical systems came motion with the report of Koshima et al., describing that thin plates For a long time, research into the photo-mechanical proper- of a crystalline amino-substituted azobenzene, when irradi- ties of azobenzene photoswitches was limited to polymeric ated with UV light, bend away from the source or radiation and liquid-crystalline materials. One potential reason for this (Fig. 2).18a The bending was accompanied with a concomitant was a general assumption, corroborated by the experimental change of the absorption profile of the crystal and roughen- and computational studies of azobenzene molecules on surfaces ing of the crystal surface, as determined by atomic force Published on 14 June 2016. Downloaded by McGill University 02/12/2016 20:21:50. Christopher Barrett (left), Oleksandr Bushuyev (centre) and Tomislav Friščić (right). At far left (in bronze), is the late James McGill (1744–1813), Montreal fur trader and University founder. Recent McGill graduate Oleksandr Bushuyev first earned BS and MS degrees at Odessa National University, Ukraine in 2007 and 2008, andthenanM.Sc.atTexasTechonaFulbrightFellowshipin2011.Hecompletedanaward-winningPh.D.onaVanierCanadaFellow- ship at McGill University in 2016, co-supervised by Barrett and Friščić, on light-responsive and photo-mechanical crystals and co-crystals. Dr. Bushuyev is now NSERC Canada Postdoctoral Fellow at the University of Toronto in Electrical Engineering. Tomislav Friščić completed a B.Sc. at the University of Zagreb, Croatia in 2001, followed by a Ph.D. at U. Iowa in 2006 on solid-state synthesis. After postdoctoral work and a Herchel Smith Fellow position at Cambridge, he joined the Faculty at McGill University in 2011, developing solid-state materials synthesis methodologies of organics, organometallics, and pharmaceuticals. McGill colleague Christopher Barrett studied at Queen's University Canada with a B.Sc. in Chemistry and Physics in 1992, and a Ph.D. in 1997, on the photochemistry of azobenzene-based polymers. After 2 postdoctoral years in Materials Engineering at MIT, he joined the Faculty of McGill in 2000, where he studies light-responsive materials for optical and bio-medical applications. This journal is © The Royal Society of Chemistry 2016 CrystEngComm,2016,18,7204–7211 | 7205 View Article Online Highlight CrystEngComm chromophores bearing ‘push–pull’-substituents, i.e. azobenzene molecules substituted on opposite ends with electron-withdrawing and -donating substituents. This substitution pattern results in a red-shifted absorbance and much shorter cis-state life- times,9 meaning that crystals of ‘push–pull’ pseudostilbenes can readily undergo photo-mechanical bending by irradiation with visible light. Moreover, the very short thermal half-lives of the photochemically generated cis-azobenzene chromophore caused the crystals to rapidly revert (within seconds) to their original shape. In all the reports described thus far, the critical parameter for effective crystal actuation was found to be the thickness of the crystal being irradiated. Overall, the best photo- mechanical responses appear to result from crystals whose thickness is in the range between 0.7 μm and 3 μm. One of the challenges in developing crystalline photo-mechanical ac- tuators is that conventional solution-based crystallization techniques often yield crystals too thick for photo- mechanical applications. A potential solution to this problem has recently been demonstrated by Lee et al., who utilized meniscus-confined solidification techniques to prepare azobenzene-containing nanowires ranging from 200 nm to 1000 nm in diameter.19

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